In the proposed study, in vivo experiments are designed: 1) To investigate the role of 5 alpha-DHT on the acquisition of fertilizing capacity of spermatozoa in the proximal corpus epididymis of the rabbit. 2) To investigate the degree of suppression of fertilizing capacity of the corpus epididymal spermatozoa by 3 (as outlined in "Background" section) 5 alpha-reductase inhibitors. 3) To determine the effect on fertilizing capacity of the cauda epididymal spermatozoa after exposing to a high 5 alpha-DHT environment. 4) To select from 3 compounds the 2 most potent ones (based on the results from (2) and 5 alpha-DHT and to explore the possibility of using these compounds in Silastic capsules to regulate the male fertility in the rabbit. 5) To monitor the changes in steroid hormones and gonadotrophins before, during and after the drug treatment to elucidate their mechanism of action in the induction of infertility and the reversibility of these treatments.